A "network" is defined herein as a particular radio telecommunication service provider, including its air interface standard. A "system" is defined herein as a combination of networks. An "air interface standard" is defined by a modulation scheme and frequency band employed by a particular network. The air interface used by various radio telecommunication networks differs. Common modulation schemes include TDMA, FDMA, CDMA, AMPS (analog cellular), and GSM. Frequency bands typically are in either the 900 MHz or 1900 MHz ranges.
As used herein, a "locational standard" or "location format" is the standard by which a particular network describes the location of a Subscriber Unit (SU). For example, some networks describe an SU location by using a latitude and longitude measurement, while other networks define an SU location by identifying a cell which the SU is located within. Networks often use differing locational standards to describe an SU location.
A variety of radio telecommunication networks have been developed to offer wider-coverage mobile communications. Terrestrial networks, such as the Global System for Mobile communications (GSM), provide communication cells to SUs using ground-based radio telecommunication equipment. More recently, satellite networks are being developed in which an SU communicates directly with a satellite which enables communications between the SU and the rest of the network.
Each communication network provides service to SUs within a particular coverage area. Terrestrial networks typically have coverage areas which are limited by physical obstructions and antenna expense. Thus, terrestrial networks are generally used in populated areas and along thoroughfares. Satellite networks are not as limited by physical obstructions or antenna expense. Thus, some proposed satellite networks intend to provide service to users across all or a substantial portion of the earth's surface.
The coverage areas of various communication networks often overlap, thus offering the potential for a particular SU to take advantage of multiple networks. For example, an SU might want to access an alternative network which offers a lower service rate, or the SU might want a call to be handed-off to another network when the SU leaves the coverage area of a particular network.
Typically, the air interfaces used by distinct radio telecommunication networks are incompatible. Most prior-art SUs can operate only with a single air interface standard. Thus, most prior-art SUs are generally capable of communicating using a single radio telecommunication network. Such a prior-art SUs would be unable to initiate a call using a TDMA network and later hand-off to a GSM network, for example.
Prior-art SUs which are capable of communicating over only a single air interface are unable to take advantage of the potential availability of multiple networks. To resolve this problem, "multi-mode" SUs are being developed which would enable an SU to operate using multiple air interfaces.
One way to enable a multi-mode SU to operate on multiple networks is described in U.S. Pat. No. 5,301,359 (Van den Heuvel et al.), which discloses a "bulletin board" communication resource which broadcasts a message describing available networks to multi-mode communication units. In response to the broadcast message, a communication unit can request, from the bulletin board, one of the available networks. The bulletin board resource then attempts to register the communication unit with the requested network and, if access to the network is allowed, sends a registration grant to the communication unit. If the communication unit can communicate using the network's RF interface, the communication unit can then begin operation using the requested network.
One problem facing inter-working networks is that locational standards are network-unique and there is no ability to translate between locational standards in the prior art. The invention described in Van den Heuval et al. does not address this problem.
Another problem not addressed by Van den Heuvel is that, when multi-mode SUs become commonplace, more networks will want to form agreements between themselves to provide service to each others' SUs. In addition, local networks are likely to experience significantly more access attempts from SUs from foreign networks. Further, networks which cover multiple geographical areas (e.g., global networks) will want to enforce possible licensing restrictions in specific geographical areas. The increase in network agreements will produce an additional level of complexity to the authentication procedure.
Because most prior-art SUs operate using a single air interface standard, prior-art SUs which use other radio telecommunication networks could easily be screened from a particular network. If an SU is incapable of using a network, the SU obviously is not authorized to use the network. However, with the advent of multi-mode SUs, SUs will not be able to be pre-screened based on the SUs ability to communicate using the network.
What is needed is a method and apparatus to enable an SU to communicate using differing networks wherein groups of unauthorized SUs are pre-screened from accessing a particular network. Further needed are SUs which can easily access multiple networks which use differing air interface standards and/or differing locational standards.